Echo cancellation (EC) is a well-known method for separation between received and transmitted signals in data transmission systems. For example, prior art echo-cancellation is described in “Echo Cancellation in Speech and Data Transmission”, David G. Messerschmitt, IEEE J., Selected Areas in Comm., Vol. SAC-2, No. 2, March 1982, pp. 283–297, which is incorporated herein by reference. A basic echo cancellation system is illustrated schematically in FIG. 1.
The system shown in FIG. 1 typically includes a Finite Impulse Response (FIR) filter which estimates the level of leakage from the modem transmit path 6 to the modem receive path 8, and is adapted for use with a modem having a transmit path 6 and a receive path 8. An echo cancellation signal, y(n), is generated by an echo cancellation signal generator 10, based on EC coefficients, Ci(n), calculated by a coefficient calculator 12. The echo cancellation filter coefficients, Ci(n), provide an estimation of the leakage function. The output, y(n), of generator 10 is received by a substracter 14 which subtracts the echo cancellation signal from the received signal, yielding an echo canceled signal, e(n), also referred to herein as residual error signal. The echo canceled signal, e(n), is fed back to coefficient calculator 12, which calculates new echo cancellation coefficients based on the residual error signal. Thus, EC coefficients Ci(n) are calculated iteratively using an adaptive process, e.g., a Linear Mean Square (LMS) algorithm, which may be executed by coefficient calculator 12. In case of a LMS adaptation algorithm, the following formula may be used:Ci(n+1)=Ci(n)+μe(n)x(n−i)  (1)                wherein:        Ci(n) is the i-th FIR coefficient at time n;        x(n) is the input to the filter at time n;        e(n) is the error at time n; and        m is the adaptation constant (controls the rate of convergence)        
Asymmetric Digital Subscriber Line (ADSL) modems are well known in the art. A basic structure of an ADSL modem is described, inter alia, in ANSI TI.413-1998—Network and Customer Installation Interfaces—Asymmetric Digital Subscriber Line (ADSL) Metallic Interface, and in T1E1.4/91-157—A multicarrier Primer, J. Cioffi, both of which are incorporated herein by reference.
Echo cancellation schemes for ADSL are described, inter alia, in Discrete Multitone Echo Cancellation, M. Ho et al., IEEE Tran. Communications, Vol. 44, No. 7, July 1996, pp. 817–825, and in U.S. Pat. No. 5,917,809, both of which are incorporated herein by reference. It should be noted that all the above mentioned schemes are based on stochastic iterative computation of the echo cancellation filter coefficients, using a residual error signal, e(n), and training sequence which is typically random, i.e., not cyclic.
FIG. 2 schematically illustrates a typical prior art echo cancellation scheme for an ADSL ATU-C modem. In this scheme, a signal generated by a constellation encoder 16 is transformed by an Inverse Fast Fourier Transform (IFFT) 18, e.g. an IFFT 512 (i.e, an IFFT having a block size of 512 samples), and transmitted via a parallel to serial (P/S) converter 20, at a rate of 2.208 Msps (Mega samples per second), to a digital to analog (D/A) converter 22, producing an analog transmitted signal 23. The signal from P/S converter 20 is decimated by a factor of 4, as indicated at block 30, and then transmitted to echo cancellation signal generator 10, which generates an echo cancellation signal as described above with reference to FIG. 1. At the receive path, an analog received signal 25 is converted by an analog to digital (A/D) converter 24 into a 552 Ksps (Kilo samples per second) digital signal. As described above with reference to FIG. 1, here too substracter 14 subtracts the echo cancellation signal, y(n), from the digital signal to yield an echo-canceled signal e(n), also referred to as “residual error signal” as described above. The echo-canceled signal is converted by a serial to parallel (S/P) converter 26 and transformed by a Fast Fourier Transform (FFT) 28, e.g., a FFT 128, to produce an output that may be fed to a frequency equalizer (not shown) followed by a constellation decoder (not shown) which reconstructs the transmitted data bits.
FIG. 3 schematically illustrates a typical prior art echo cancellation scheme for an ADSL ATU-R modem. In this scheme, a signal generated by a constellation encoder 116 is transformed by an IFFT 118, e.g. an IFFT 128, and transmitted via a parallel to serial (P/S) converter 120, at a rate of 552 Ksps, to a digital to analog (D/A) converter 122, producing an analog transmitted signal 123. The signal from P/S converter 120 is received by echo cancellation signal generator 10 which generates an echo cancellation signal, as described above with reference to FIG. 1. The echo cancellation signal, y(n), is interpolated by a factor of 4, as indicated at block 132, before being received by substracter 14. The interpolation process adds a sufficient amount of interpolated data to the echo cancellation signal, thereby to increase the transmission rate by a desired factor, e.g., by a factor of 4. Such an interpolation method is well known in the art and is described, for example, in “Digital Signal Processing Schemes for Efficient Interpolation and Decimation”, R. A. Valenzuela, IEEE Proceedings, Vol. 130, No. 6, pp. 225–235, December 1983. At the receive path, an analog received signal 125 is converted by an analog to digital (A/D) converter 124 into a 2.208 Msps digital signal. Substracter 14 subtracts the interpolated echo cancellation signal from the digital signal to yield an echo-canceled (residual error) signal e(n). The echo-canceled signal is decimated by a factor of 4, as indicated at block 130, and the decimated signal is fed back to coefficient calculator 12. The echo-canceled signal is converted by a serial to parallel (S/P) converter 126 and then transformed by a Fast Fourier Transform (FFT) 128, e.g., a FFT 512.